The invention relates to a method for reducing the wheel slip of a motor vehicle in accord with the generic concept of claim 1.
When motor vehicles drive through a curve, those wheels on the inside of the curve run on a smaller radius of curvature than do the wheels on the outside of said curve. Since the inside wheels travel, on that account, over a shorter distance than the outer wheels, their circumferential velocity and thereby their speed of rotation is correspondingly less. In order to enable this condition to take place, the wheels of a driven motor vehicle axle, with the exception of independent suspension drive vehicles, are powered through a differential drive. By this means, the wheels on one axle can roll over the road without wheel slip relative to the pavement. In this way, the driving force of the wheels of one axle support one another on the road. If a wheel should spin, then wheel slip is generated, relative to the road and the support torque correspondingly reduces itself to a torque which is a function of the friction load of the spinning wheel. The forward drive torque of the other wheel correspondingly lessens itself so that the traction of the motor vehicle is substantially reduced. Furthermore, because of the wheel slip, the lateral grip of a wheel and, in combination therewith, the stability of the motor vehicle is substantially impaired.
In order to improve the traction and also very frequently tocarry out dynamic corrections in driving, differential drives with electronically limited slip-control find application. Further, the brakes in the individual wheels are specifically controlled in order to bring about a positive influence on the behavior of traction and stability of the vehicle.
Where the said regulations are concerned, the wheel slip serves as an control-input value. That is, if a wheel slipped so severely, that the differential slip control or a brake could lessen the slip, then the regulation would be activated. For this purpose, the wheel slip must be defined. In existing systems, the reference speed of the motor vehicle was determined in relation to the road.
By means of the reference speed, the desired speed, that is to say, the rotational speed of the wheels is computed and compared with the actual circumferential speed, i.e., speed of rotation of the wheels. The reference speed must be known as exactly as possible. The measurement or the computation of the vehicle reference velocity is very laborious and time consuming. In the case of 4-wheel drive vehicles, a special sensor must be used or the reference speed empirically estimated by means of various procedures.
Thus, the invention has the purpose to improve, with simple measures, the traction and travel stability of a motor vehicle without the necessity of knowing the reference speed of the motor vehicle. This purpose, in accord with the invention, is achieved by the features of claim 1.
In accord with the invention, the speeds of rotation of the wheels are determined. These are, in a first approximation, representative of the circumferential speed of the wheels since the speeds of rotation of the wheels, in general, stand in a linear relationship with the circumferential velocities. A virtual curve radius can be calculated from the speeds of rotation of the wheels on the right vehicle side of the axle and the wheels on the left vehicle side of the axle, by the use of geometric motor vehicle parameters, among these being especially the wheel gauge. The center of the axle would follow this virtual curve radius, if it were not coupled to the motor vehicle and if the wheels exhibited no slippage. Further, a desired radius is determined on the basis of vehicle parameters, especially giving consideration to the wheel base of the vehicle axles. The average steering angle of the wheels is found through the average value of the steering angle of the outside curve wheel and that of the inside curve wheel, both wheels being on the same axle. The midpoint of the axle would move itself on such a curve, if the wheels were rolling without slippage.
In an electronic control unit, which advantageously includes a microprocessor, the virtual curve radius is compared with the desired radius. If the wheels have no slip, then the desired radius and the virtual radius coincide throughout. From the result of this comparison of the virtual with the desired radius, a slip-related characteristic value is generated which-serves the purpose of correspondingly activating and controlling an associated brake or differential slip control in the differential drive itself.
Advantageously, the said characteristic value incorporates the ratio, or else the difference of the desired radius to the virtual radius, which is indeed advantageous from the standpoint that the activation can only be brought about, when the desired radius is greater than the virtual radius. In other words, in this case, the ratio of desired radius to virtual radius is greater then one. To establish the characteristic value for the control of the differential slip control or, in some cases, control of a brake, it can be additionally advantageous to give consideration to the change of the result over time and/or the change of other parameters, these being yaw velocity, and/or transverse acceleration as well as the driving torque of the motor vehicle. In this way, the traction and the driving stability of a vehicle is brought into a balanced relationship.
In the Description and in the Claims, a multitude of features are presented and described in connection with one another. The expert can observe the combined features advantageously within the concept of a purpose to be achieved as well as in their individuality and be able to put them together in logical additional combinations.